A NOTE ON THE ORIENTATION OF THE MACRODISPERSION TENSOR IN PARTIALLY SATURATED HETEROGENEOUS POROUS FORMATIONS

The general situation in which the mean gradient vector, J, is not nec essarily aligned in a direction perpendicular to the bedding of a part ially saturated heterogeneous formation, is considered. Previous resul ts of first-order analyses, based on a stochastic continuum presentati on of unsaturated flow and a general Lagrangian description of the tra nsport(15) and standard tensor transformation relationships, were used to evaluate the orientation of the principal axes of the asymptotic m acrodispersion tensor as a function of the inclination of J, and to as sess its impact on solute spreading in partially saturated heterogeneo us formations of three-dimensional anisotropic structure. The main res ults obtained in this study suggest that under unsaturated flow, for g iven statistics of the formation properties and given mean water satur ation, and when the inclination of J is relatively small, coarse-textu red soil material with negligibly small capillary forces may compensat e for the enhanced lateral displacement (in the same direction as that of J relative to the longitudinal axis of the formation heterogeneity ) and the enhanced lateral spreading of the solute plume due to increa sing anisotropy (smaller e = I-yv/I-yh where I-yv and I-yh are the cor relation length scales of log-unsaturated conductivity in the vertical (longitudinal) and horizontal directions, respectively). On the other hand, when the inclination of J is relatively large, such soil materi al may enhance lateral displacement of the solute plume in a direction opposite to that of J relative to the longitudinal axis of the format ion heterogeneity. (C) 1997 Elsevier Science Ltd.